1. Field of the Invention
The present invention relates to an optical scanning apparatus. Specifically, the invention relates to an optical scanning apparatus in which a plurality of light beams different in wavelength are guided through a set of scanning means (scanning optical system) onto one or more planes to be scanned, on a recording medium (for example photosensitive drum) to conduct simultaneous optical scanning.
2. Related Background Art
Among recent digital copiers is a so-called dichromatic color copying laser beam printer of multiple exposure and multiple development in which at least two different regions on a plane to be scanned are simultaneously exposed to laser beams (light beams) and in which the respectively exposed regions are developed by respective developing devices different in color.
In the dichromatic copying laser beam printer, the two regions on the plane to be scanned are simultaneously optically scanned by a device structured to have two light beams and two sets of scanning means (scanning optical systems).
FIG. 1A and FIG. 1B are schematic drawings of main part to show a conventional optical scanning apparatus to perform the simultaneous optical scanning of two regions on a surface to be scanned using the scanning optical systems. FIG. 1A is a main part side view to show a part of the optical scanning apparatus as seen along a direction perpendicular to a rotation shaft of a light deflector, and FIG. 1B a main part plan view as seen along a direction of the rotation shaft of the light deflector.
In FIG. 1B one light source means 51a comprising a semiconductor laser emits a light beam toward a collimating lens 52a, and the collimating lens 52a collimates the light beam into a parallel light beam. The collimated light beam passes through a cylindrical lens 53a having a predetermined refractive power in a direction normal to a sheet plane of FIG. 1B, that is, in a sub scanning direction thereby to form a linear image near a reflection plane (deflection plane) 54a of a light deflector 54 comprising a polygon mirror.
Another light source means 51b emits a light beam toward a collimating lens 53b, and the collimating lens 52b collimates the light beam into a parallel light beam. The collimated light beam passes through a cylindrical lens 53b having a predetermined refractive power in the sub scanning direction thereby to form a linear image via a reflection mirror 55 near a reflection plane 54b of a light deflector 54.
In FIG. 1B, an optical path from the light source means 51a to the reflection plane 54a of the light deflector 54 is shifted in the direction normal to the sheet plane of FIG. 1B with respect to an optical path from the light source means 51b via the reflection mirror 55 to the reflection plane 54b, whereby they are different in height.
The respective light beams are reflection-deflected by the light deflector 54 having the upper and lower two level reflection planes (deflection planes) 54a and 54b. The light deflector 54 is rotated by a motor 56 of drive means at a certain speed in a direction of arrow A.
The respective light beams reflection-deflected by the respective reflection planes 54a, 54b thereafter pass through an image forming optical system 57 having the f-.theta. characteristics, which comprises two lenses 57a and 57b. Each of the two lenses 57a and 57b is comprised of upper and lower two levels adhered to each other. The light beam reflection-deflected by the reflection plane 54a passes through the upper level of the image forming optical system 57 in FIG. 1A, and is imaged (converged) via turn mirrors 60, 61 at an exposure position P2 on a photosensitive drum 62.
On the other hand, the light beam reflection-deflected by the reflection plane 54b passes through the lower level of the image forming optical system 57 as shown in FIG. 1A, and is imaged (converged) via turn mirrors 58, 59 at an exposure position P1 on the photosensitive drum 62.
Optical scanning is carried out at the respective exposure positions P1, P2 in a direction normal to the sheet plane of FIG. 1A (in the main scanning direction) by rotating the light deflector 54 in the direction of arrow A. The photosensitive drum 62 is rotated in the sub scanning direction as shown by an arrow B with the exposure in the main scanning direction, whereby the photosensitive drum 62 is sequentially exposed at the exposure positions P1, P2. Electrostatic latent images on the photosensitive drum 62 are developed by developing devices (not shown) corresponding to the respective light beams.
The conventional optical scanning apparatus employs two sets of scanning means (scanning optical systems) as described for simultaneous optical scanning of two regions on a plane to be scanned using the two light beams. Such an arrangement requires a large space for installation of the scanning means, resulting in increase in size of the entire apparatus and in the number of parts whereby to increase a production cost thereof.
In addition, the polygon mirror as the light deflector is structured to have the upper and lower two levels of reflection plane as shown in FIG. 1A, or, the reflection plane of the polygon mirror must be enlarged in the sub scanning direction. Thus, an occupying space of the polygon mirror increases in the apparatus, and loads on a motor also increase to obtain a certain rotation speed, which are also problematic in the conventional apparatus.
Further, in practice, a gap between the light beams is restricted in respect that the two light beams are to be reflected by the respective reflection planes 54a, 54b, so that it becomes very difficult to arrange the turn mirrors in position.